The present invention relates to an arc extinguishing aid with an externally powered magnetic blowout coil for switching low currents.
DC power switches such as tie circuit-breakers, section switches and rectifier switches are used for a variety of tasks in DC systems of electric railways etc., where a variety of requirement profiles are encountered with respect to carrying and switching in two current directions. Tie circuit-breakers must carry and switch currents in both directions, while section switches and rectifier switches may pass current in the direction opposite to the normal current direction during regeneration, and the switches must therefore be capable of switching certain current levels of up to 5 kA in both current directions.
In large DC power switches, extremely long turn-off times can occur during the switching of critical or low currents in the range from 0 to 400A. These extremely long turn-off times occur during switching via shunt trips in normal operation and cause a long arc lifetime, which in turn can cause severe wear in the contact system of the switching device, and damage the switching device itself.
The cause for the long turn-off times resides among other factors in the small electromagnetic force at low current which drives upward and elongates an arc resulting from a switching process. The arc can be viewed as a current-carrying conductor on which a force is exerted when a magnetic field is encountered. When the self-field of the current-carrying conductor is small, an external magnetic field can be applied to increase arc movement. This field can be implemented via magnetic blowout coils wired in series in a primary circuit, for example. These coils either pass the primary current continuously or during an extinguishing process, for example, when probes are connected to the primary circuit via coils.
However, these solutions have a disadvantage to the effect that the coils switched into the primary circuit must have a high current-carrying capacity and dynamic strength since the primary current pauses through them.
Other alternatives, e.g., a field implemented with the use of a permanent magnet or externally powered coils, can only be effective in one predefined current direction. In the opposite current direction, they would force the arc into the switch, thereby destroying the switch. However, since currents must be switched in both directions as discussed above, such alternatives are not suitable choices for solutions.
The above-described drawbacks and deficiencies of the prior art are overcome or alleviated by a coil that is powered by a control supply network and generates a magnetic field whose field lines penetrate that arc vertically and whose field direction is changed by the control unit based on the circuit for detection of the current direction. The magnetic field, in conjunction with the arc current, causes an elongation of the arc and drives it from the contacts into the arc chamber.
Advantageous embodiments of the invention are characterized in the dependent claims.
The magnitude of the magnetic field generated by the externally powered coil is independent of the current magnitude of the primary circuit. Consequently, when currents are very low, for example 1A, a sufficient magnetic field is available for the elongation. There is no need for coils that have a high short-term current carrying capacity and are expensive, and moreover, no thermal losses occur due to elements wired into the primary circuit.
The invention is described in greater detail below using an example embodiment and with reference to the drawings.